1. Field of the Invention
The present invention is directed to a device for generating a magnetic field in an air gap, particularly for a magnetic resonance apparatus, of the type having a pair of magnetic poles arranged disposed opposite one another between which the air gap is located, each of the magnetic poles having a pole plate constructed of a number of layers lying on top of one another and electrically insulated from one another.
2. Description of the Prior Art
A device of the type described above is disclosed in European Application 0 479 514. A magnetic field having high field strength and which satisfies high demands as to homogeneity is generated with a pair of magnetic poles arranged lying opposite one another, the field being generated in an air gap lying therebetween. Such a device is used for a diagnostic magnetic resonance apparatus. The air gap is generally of such a size that a patient for examination can be placed between the poles after the installation of the other components. The magnetic poles also include a magnetic drive, for example in the form of permanent magnets or electromagnetic coils as well. For the flux return, the magnets are connected via a yoke composed of highly magnetically permeable material.
In imaging with magnetic resonance, gradient coils are required that are secured to the magnetic poles and that generate respective magnetic gradient fields in the x-direction, y-direction and z-direction. Location information is superimposed on the magnetic resonance signal by means of these magnetic gradient fields. The magnetic gradient fields are usually generated by current pulses that have the side-effect of inducing eddy currents in adjoining, electrically conductive regions. The induced eddy currents in turn distort the gradient field in the air gap.
For reducing these induced eddy currents, the magnetic poles disclosed in European Application 0 479 514 have pole plates that are constructed of individual blocks, the blocks themselves being composed of a number of electrically insulated layers of non-oriented silicon steel sheet. This structuring is generally adequate, but the fabrication is complicated. The blocks must be individually mounted on a carrier, which can only thereafter be connected to the magnetic pole. The magnetic properties are not optimum due to the small gaps between the individual blocks.
European Application 0 691 548 discloses that plate-shaped "soft ferrites" also can be employed in pole plates.
U.S. Pat. No. 5,317,297 discloses pole plates for the basic field magnet of a magnetic resonance apparatus that are composed of relatively large rod-shaped elements. It is stated therein that even a single layer composed in this way effects a significant reduction of eddy currents. Two layers that are orientated 90.degree. relative to one another, however, are more mechanically stable and tend toward a more uniform distribution of the eddy current effects.